Papers by Author: Chang Pin Chou

Abstract: Femtosecond laser (FS-laser) microstructuring of metals has become a promising tool because of its non-contact nature, which allows the micromachining and direct processing of materials with a minimized volume of heat-affected zone for electro-optics applications such as light emitting diodes (LED) and solar photovoltaic (PV) lighting. This study presents ultra-short pulse (10^-15 sec) FS-laser processing. Through integrating the laser source, optical system and dynamic control modules, the materials of metals with micro-scale or nanoscale structures can be fabricated. In traditional processing such as semiconductor processing, development, exposure and etching necessitate expensive equipment and time-consuming tasks. With FS-laser processing, high-precision patterns are obtained, which will be a great benefit to keeping costs down. In this study, the wavelengths of FS-laser ablation are employed using visible and infrared light. To make a breakthrough in electro-optics processes, the CIGS thin-film of solar cells in metal process can be easily produced by the FS-laser. The ablation speed of the FS-laser for thin film layer CIGS solar cells can reach 2000 mm/s which is faster than the current Nd:YAG laser machine (~1000 mm/s). On the other hand, the minimum size of metal lines can be controlled to a value that is lower than 40 µm. Furthermore, green energy can be effectively developed for the future.

Abstract: This paper presents an approach to utilize high precision pulsed Nd:YAG laser to fabricate a rough array-pattern on a soda-lime glass plate by a laser-induced backside writing (LIBW) process, and a laser-induced plasma assisted ablation (LIPAA) technique. The current study investigates the effect of process parameters such as single-shot laser exposure time and number of passes on the material removal rate. After depositing 695 nm thick Teflon thin film on the glass plate, the surface of the laser micro-machined template becomes hydrophobic. The surface roughness, annular groove profile and surface micrograph were measured by an atomic force microscope, a profilometer, and a scanning electron microscope, respectively. A uniform liquid droplet by the sessile drop method is generated on the hydrophobic template. Droplet characteristics, such as contacted angle, size, and shape, are measured with a surface tension analyzer and microscope. This work also discusses the relationship between the formed droplets and the process recipe of the micro-machined template. The proposed approach can apply to future for uniform lens array formation.

Abstract: This paper proposed a novel method to fabricate micro-cylindrical lens array (μ-CLA) by using wire electrical discharge machining (WEDM) and precision glass molding (PGM) technique. Micro slits fabricated on the surface of tungsten carbide mold material with outline dimensions from 0.3mm to 2.1mm, the width and depth of 1 mm. The PGM processing parameters including width of slits, molding temperature, molding force and stroke were discussed. Moreover, the characteristics of μ-CLA were measured by contact profilometer. In addition, the relationship between the dimensions of the slit on tungsten carbide mold and the characteristics of μ-CLA also discussed.

Abstract: The purpose of the present work was to investigate the effect of oxide fluxes on surface appearance, weld morphology, angular distortion, and weld defect obtained with activated tungsten inert gas (TIG) process applied to the welding of 6 mm thick dissimilar metal plates between JIS G3131mild steel and SUS 316L stainless steel. The CaO, Fe2O3, Cr2O3, and SiO2 fluxes used were packed in powdered form. The results indicated that the surface appearance of TIG welds produced with oxide flux formed residual slag. TIG welding with SiO2 powder can increase joint penetration and weld depth-to-width ratio, and therefore the angular distortion of the dissimilar weldment can be reduced. Furthermore, the defects susceptibility of the as-welded can also be reduced.

Abstract: The effect of dissimilar welding of 7050/2024 aluminum alloy on mechanical properties and microstructure were analyzed and investigated. The post-welding heat treatments of weldments were carried out and recorded for microstructure, tensile test, hardness and fracture surface in different conditions. The experimental results indicate that the post-welding heat treatment, filler metal chose can increase the mechanical properties. The microstructure and fracture surface of dissimilar welding can evaluate the distribution of precipitation strengthening phase in fusion zone, heat affected zone and its fracture type.

Abstract: This paper describes a heat treatment process using dual aging for the A7050 aluminum alloy with the Taguchi method to optimize process parameters. The current study considers micro hardness and electrical conductivity as optimization criteria. Pre-aging temperature, pre-aging time, re-aging temperature, and re-aging time are important factors influencing these optimization criteria. Experiment results show that re-aging temperature is the most significant parameter for electrical conductivity, and both aging times are important influence factors for micro hardness performance. The optimal hardness for A7050 heat treatment conditions are pre-ageing temperature 120 °C, pre-ageing time 12hrs., re-aging temperature140 °C, and re-aging time 8 hrs., respectively. The best electrical conductivity parameters are pre-ageing temperature 120 °C, pre-ageing time 4 hrs., re-aging temperature180 °C, and re-aging time 24 hrs., respectively. The current study obtained contributing individual parameters for hardness, and electrical conductivity in dual aging of heat treatment.

Abstract: This paper applies an integrated approach using the Taguchi method, neural network (NN) and genetic algorithm (GA) to optimize the tensile-shear strength of resistance spot welding (RSW) specimens in automotive industry. The proposed approach consists of two stages. First stage executes initial optimization via Taguchi method to construct a database for the NN. In second stage, a NN with Levenberg-Marquardt back-propagation (LMBP) algorithm is used to provide the nonlinear relationship between factors and the response. Then, a GA is applied to obtain the optimal factor settings. The experimental results showed that the tensile-shear strength of the optimal welding parameter via the proposed approach is better than apply Taguchi method only.

Abstract: The objective of this study is to explore the influence of the steel’s material properties on the distribution of welding residual stresses of the butt-welded steel plate. First, S15C carbon steel’s material properties are varied in this study to calculate the corresponding welding residual stress by carrying out 3D finite element analyses. Results show the maximum longitudinal tensile residual stresses increase linearly with increasing thermal expansion coefficient and increasing yield stress. The length of the tensile residual stress zone decreases linearly with increasing thermal conductivity. Finally, based on the relationship between material properties and welding residual stresses, a useful prediction equation is developed and verified in this study.

Abstract: In order to achieve single pass welds without edge preparation, instead of multipass procedures, one of the most notable techniques is to use activating flux with GTA welding (GTAWflux) process. In autogenous GTA welding mixed fluxes (oxide SiO2 and Cr2O3), were applied on the weld area of type 310 stainless steel through a layer of the flux to produce a bead-on-plate weld. Many parameters affect the GTAW-flux process quality. The Taguchi method can improve on the disadvantages of full factorial design. However, in practice, it has certain limitations. In this work, a Taguchi-Neural approach (Combine Taguchi method and a neural network) was used to select the GTAW-flux process parameters. The experimental procedure of GTAW-flux process, via the proposed approach produced full penetration of the weld root in 8 mm thick type 310 stainless steel with single pass welds.

Abstract: The laser ablation technique is one option for micro-machining and patterning of diamond film. A UV YAG laser with higher energy density can remove or destroy the diamond film more efficiently than the excimer laser. That is, the UV YAG laser not only provides faster etching rate on the diamond film, but also requires less processing and maintenance cost. In the current study, synthetic diamond films with grain size of 30 μm were deposited on silicon substrate by microwave plasma enhanced chemical vapor deposition (MPCVD) in the CH4/H2 mixture atmosphere. A pulsed UV YAG laser (λ = 355 nm, 10 kHz) was employed to machine and remove the diamond film. The diamond film surface was analyzed by SEM and Raman spectroscopy after the laser machining. The beam size of YAG laser was adjusted to between 0.1 mm and 1.5 mm by the trepan mechanism to approach the following defined scanning width. In order to shape a 4-inch diamond wafer into a microstructure, the scanning width of the UV YAG laser was defined to 0.1 mm, 0.75 mm and to 1.5 mm in several loops. The results show that the laser-polishing effect can be applied to the pretreatment of mechanical polishing of diamond wafer in the condition of 0.75 mm scanning width in 3 loops. From Raman spectrum, it could prove the mechanism of carbon burning reaction during the laser processing and the residual carbon existing in the laser-patterned area. The surface of diamond film is strongly affected by the laser processing and a better result from the parameter of 0.75 mm scanning width in 3 loops is shown in the current study.